The present invention relates to a method of obtaining an image of an underground formation around a borehole extending through the underground formation. The image that is to be obtained comprises a set of reflectors attributed to underground positions in the underground formation around the borehole. Such an image is produced to provide detailed information of the underground formation while drilling the borehole. This information allows planning the direction into which the borehole is drilled. This is particularly useful when it is required that a horizontal borehole that is being drilled is kept within a thin formation layer.
The method and apparatus of the present invention calls for the activation of a tool having an omnidirectional low frequency source in the well borehole. The energy travels into the formation and is reflected back towards the tool where it is detected by a three component receiver. The return signal is then recorded for further processing. The return data may be processed to determine ray direction and two-way travel times for the return data utilizing known techniques. A point is then selected in the formation. These points may be selected based on prior seismic data or core sampling with respect to velocity times in the formation. Accordingly, the present method utilizes some a priori knowledge with respect to expected return times for the formation. For the selected point, a hypothetical or projected ray direction and travel times are calculated using the same techniques The projected ray and travel times are compared with a calculated ray/travel time. If the parameters are sufficiently similar, the data is attributed to the point. If the calculated and projected ray and travel times are not sufficiently similar, a new point in the formation is selected, projected ray and travel times computed and are once again matched against calculated data. This process is repeated until all calculated ray and travel times have been attributed to various point reflectors in the formation. Using this information, an image of the formation may be created using the various point reflectors.
In the specification and in the claims the term xe2x80x98two-way travel timexe2x80x99 is used to refer to the time it takes for seismic energy to go from a source via a reflector to a receiver.
It will be understood that in order to perform the calculations in step (e), it is required to know the seismic velocities in the formation. These seismic velocities can be obtained from previous seismic work done in relation to the formation, or they can be obtained from core samples. In addition, sonic measurements can provide information on the seismic velocities.